Up to now, diverse types of fuel cells have been researched and developed.
Among those fuel cells, a polymer electrolyte fuel cell has been extensively researched and developed as an on-vehicle or household generating equipment for the reason that the polymer electrolyte fuel cell is easily handled because an operating temperature is relatively low, and an electrolyte is formed of a polymer membrane.
On the other hand, for the purpose of carrying on a small-sized electric equipment in use, diverse primary batteries and secondary batteries have been employed. However, with increasing performance of the recent small-sized electric equipment, a power consumption is increased, and the primary battery that is small in size and light in weight is incapable of supplying a sufficient energy.
Also, the secondary battery is advantageous in that the secondary battery can be repetitively charged in use, but an available energy that can be charged by one charging is still smaller than that of the primary battery.
In order to charge the secondary battery, another power source is required, and it normally takes several tens minutes to several hours to charge the secondary battery. Thus, it is difficult to enable the secondary battery to be soon used anytime and anywhere.
In the future, the electric equipments are increasingly reduced in size and weight, and the wireless network environments are prepared, so there is an increasing tendency to carry on the equipment in use. However, the conventional primary cells and secondary cells are difficult to supply the sufficient energy for driving the equipments.
In order to solve the above problem, attention has been paid to the small-sized fuel cell. This is because the available energy quantity per volume or per weight is several times to ten times as large as that of the conventional fuel cell on the ground that the fuel cell is valuable to the driving source of the small-sized electric device.
In addition, because the small-sized fuel cell can be continuously used by exchanging only a fuel, it takes no time for charging unlike in the case of the secondary battery. The small-sized fuel cells that are mainly used are of the polymer electrolyte type or the direct methanol type.
In the polymer electrolyte fuel cell, a polymer electrolyte membrane is used as the electrolyte, and a membrane electrode assembly has catalyst electrode layers on both sides thereof. A fuel (hydrogen) is supplied to one of the catalyst electrode layers (anode), and an oxidizer (air) is supplied to another catalyst electrode layer (cathode) to conduct power generation.
In this case, water is generated as a product.
Reaction formulas at the anode and the cathode are represented as follows.Anode: H2→2H++2e31Cathode: ½O2+2H++2e31→H2O
The theoretical voltage of a pair of membrane electrode assembly is about 1.23 V, but in many cases, the voltage is about 0.7 V in a normal operating state.
For that reason, when higher voltage is required or higher output density is required, a plurality of fuel cell units are laminated, and the respective fuel cell units are electrically connected in series with each other in many cases.
The above laminated structure is called “fuel cell stack”, and in general, an anode flow path and a cathode flow path are isolated from each other by a member that is called “separator.”
In the following description of the present invention, the fuel flow paths denote flow paths in which the fuel that is supplied from a fuel container circulates within the fuel cell system.
That is, the fuel flow paths denote a flow path for guiding the fuel from the fuel container to the fuel cell, a flow path for supplying the fuel to the anode in the fuel cell, a flow path that is provided in the anode, and a flow path that extends up to a exhaust mechanism for exhausting the fuel within the fuel cell from the fuel cell to the external.
In particular, the flow path within the anode is called “anode flow path,” or merely “anode.”
During the power generation of the fuel cell, impurity gas, which is attributable to the power generation, such as nitrogen in the air or the generated moisture vapor is gradually stored within the fuel flow path, because the electrolyte membrane that is used in the polymer electrolyte fuel cell penetrates a slight amount of air.
In particular, in the fuel cell of the circulation type or the dead end type which is high in the fuel utilization rate, the power generation characteristics of the fuel cell are deteriorated by the aid of the stored impurity gas.
For that reason, Japanese Patent Application Laid-Open No. 2004-171967 discloses the fuel cell of the dead end type in which a purge valve is provided in the fuel flow path, and the purge operation is conducted during the power generation to prevent the characteristics from being deteriorated.
The purge operation is the operation of purging the impurity gas by the aid of the fuel gas. For that reason, not only the impurity gas but also the fuel gas is contained in the exhaust gas.
In a case of using hydrogen as the fuel, attention is paid so that the ratio of fuel to air does not fall within 4 to 75%.
Under the circumstances, in Japanese Patent Application Laid-Open No. 2003-132915, the following proposal is made in order to dilute the exhaust fuel to a concentration other than the above range.
In Japanese Patent Application Laid-Open No. 2003-132915, there is proposed, the fuel cell system in which the purged fuel gas is diluted by the aid of a cathode off gas that is exhausted from the fuel cell within a diluter.
Also, in Japanese Patent Application Laid-Open No. 2005-108805, there is proposed that the fuel gas is mixed with the cathode off gas and is burned using the catalyst, to thereby exhaust the fuel gas after dilution.
In the catalytic combustion system of the above type, in particular, it is necessary to stably maintain a blaze when the fuel gas is diluted by combustion.
In order to meet the above requirement, in Japanese Patent Application Laid-Open No. 2006-183977, there is proposed that a volume chamber that acts as a buffer is provided in front of a combustion chamber to accumulate the purge gas, to thereby supply a given quantity of fuel to the combustion chamber through intermittent purge operation.
Also, Japanese Patent Application Laid-Open No. 2006-183977 discloses a catalyst combustion chamber in which a current plate that supplies a pressure loss to the mixture gas is provided to the upstream side of a combustion portion in order to sufficiently mix the fuel with an oxidizer. In addition, the current plate has a flow path diameter that is equal to or less than a quenching diameter of the fuel, to thereby prevent the blaze within the combustion chamber from being transmitted upstream.
In general, the ratio of the surface area to the volume is large in a fine space, and a heat is easily escaped. Accordingly, an area where the blaze cannot be maintained in the mixture blaze within the fine space because the combustion heat is liable to be taken by an outer wall. This phenomenon is called “quenching phenomenon.” For example, when an interval between the two parallel walls becomes equal to or less than a given distance, the blaze cannot be propagated. The limit value is called “quenching distance,” and also “quenching diameter” in a case where the flow path is a circular tube. The quenching distance and the quenching diameter are different depending on the gas, for example, the quenching distance of hydrogen is 0.51 mm. Accordingly, in order to stably maintain the blaze, a space that is equal to or more than the quenching distance is required. Conversely, in order to prevent the flashback, a filter (quenching filter) having a filter diameter which is equal to or less than the quenching diameter is used.
However, the above-mentioned conventional diluter for diluting the exhaust fuel to the concentration other than the above range has the following problems.
For example, in the above conventional fuel cell system disclosed in Japanese Patent Application Laid-Open No. 2003-132915, it is difficult to obtain a sufficient flow rate in a natural diffusion because the cathode off gas that is supplied by a pump or a blower is used for dilution.
Also, in order to control the fuel gas flow rate, because the flow rate control valve and a circuit for controlling the flow rate control valve are required, the volume becomes large, and an electric power for control is required.
Also, in the above conventional fuel cell systems disclosed in Japanese Patent Application Laid-Open Nos. 2005-108805 and 2006-183977 where the fuel gas is diluted by the catalyst combustion system, there arises such a problem that a temperature locally increases because the blaze is generated in the combustion chamber.
In addition, in order to dilute the fuel gas while maintaining the stable blaze, as is disclosed in Japanese Patent Application Laid-Open No. 2006-183977, it is necessary that the current plate using another part be provided on the upstream side of the combustion portion to supply the pressure loss to the mixture gas, to thereby precisely control the supply quantity of the fuel and the oxidizer.
In particular, in a case of the small-sized combustion chamber, a countermeasure against the quenching phenomenon is required.